[tz] Some time zone history (2)

[Tim Parenti]

On Fri, 27 Nov 2020 at 07:04, P Chan <legco at outlook.com> wrote:

> Turks and Caicos Islands
>
> Standard Time Declaration Order 2015 (L.N. 15/2015)
> http://online.fliphtml5.com/fizd/czin/#p=2
>
> Standard Time Declaration Order 2017 (L.N. 31/2017)
> http://online.fliphtml5.com/fizd/dmcu/#p=2


Thanks; I've installed the attached patch.  Although this doesn't change
any historical clock offsets, it does model the period 2015-03-08/11-01 as
AST, rather than as EDT, which also changes the DST flag.

As for the 2018 change… Although L.N. 31/2017 reads that it "shall come
into operation at 2:00 a.m. on 11th March 2018", a precise interpretation
here poses some problems.  The order states that "the standard time to be
observed throughout the Turks and Caicos Islands shall be the same time
zone as the Eastern United States of America" and further clarifies "[f]or
the avoidance of doubt" that it "applies to the Eastern Standard Time as
well as any changes thereto for Daylight Saving Time."  However, as clocks
in Turks and Caicos approached 02:00 -04, and thus the declared
implementation time, it was still 01:00 EST (-05), as DST in the Eastern US
would not start until an hour later.

Since it is unlikely that those on the islands switched their clocks twice
in the span of an hour, we've assumed instead that the adoption of EDT
actually took effect once clocks in the Eastern US had sprung forward, from
03:00 -04.  This discrepancy only affects the time zone abbreviation and
DST flag for the intervening hour, not wall clock times, as -04 was
maintained throughout.

--
Tim Parenti
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